Step attenuator of low inductance and high bandwidth

ABSTRACT

A drum attenuator for selectively connecting a plurality of attenuator sections of different value into an electrical circuit is described, in which the selected attenuator section is grounded through at least two adjacent parallel current paths of approximately equal length for each ground point to reduce inductance and to prevent signal distortion due to &#39;&#39;&#39;&#39;ringing.&#39;&#39;&#39;&#39; The attenuator sections are each in the form of a ceramic strip having a pair of voltage divider resistors and a damping resistor coated thereon and having capacitors formed integrally therewith. The attenuator sections are releasably mounted within slots around the periphery of the attenuator drum by metal clips which also serve as ground planes. The resulting step attenuator apparatus has an extremely wide frequency bandwidth and may be used in a high-frequency cathode-ray oscilloscope to attenuate its vertical input signal by a plurality of predetermined steps.

United States Patent [72] Inventor Darrell. Wilhoit Beaverton, Greg.[2]] Appl. No. 830,071 [22] Filed June 3, 1969 [45] Patented Nov. 23,1971 [73] Assignee Tektronix, Inc.

Beaverton, Oreg.

[54] STEP A'ITENUATOR 0F LOW INDUCTANCE AND HIGH BANDWIDTH l 1 Claims, 7Drawing Figs.

[52] 11.5. C1 333/81, 333/81 A, 333/97 [51] Int. Cl 1101p l/22 [50]Fieldof'Search 333/81,81 C, 81 A, 97

[561- References Cited UNITED STATES PATENTS 3,215,959 11/1965 Velsink333/81 3,219,953 11/1965 Schwartz 333/81 3,299,373 1/1967 Conney v.333/81 Assistant Examiner-Saxfield Chatmon, Jr. Attorney-Buckhorn,Blore, Klarquist and Sparkman ABSTRACT: A drum attenuator forselectively connecting a plurality of attenuator sections of difierentvalue into an electrical circuit is described, in which the selectedattenuator section is grounded through at least two adjacent parallelcurrent paths of approximately equal length for each ground point toreduce inductance and to prevent signal distortion due to ringing." Theattenuator sections are each in the form of a ceramic strip having apair of voltage divider resistors and a damping resistor coated thereonand having capacitors formed integrally therewith. The attenuatorsections are releasably mounted within slots around the periphery of theattenuator drum by metal clips which also serve as ground planes. Theresulting step attenuator apparatus has an extremely wide frequencybandwidth and may be used in a highfrequency cathode-ray oscilloscope toattenuate its vertical input signal by a plurality of predeterminedsteps.

PATENTEDHBV 23 ml 3,622,919

SHEEI 2 [IF 2 FIG. 6

56 58 6O 9 62 64 5 INPUT OUTPUT y II I I DARREL I. WI LHOIT INVENTORBUCKHORN, BLORE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS STEP ATTENUATOR F LOWINDUCTANCE AND HIGH BANDWIDTH BACKGROUND OF THE INVENTION The subjectmatter of the present invention relates generally to step attenuatorsfor varying the amplitude of an electrical signal in predetermined stepsby selectively connecting a plurality of attenuator sections ofdifferent value into the electrical circuit through which such signal istransmitted. More particularly, the present invention relates to drumattenuators having a plurality of attenuator sections mounted on arotatable drum, of the type shown in Us. Pat. No. 3,074,01 l of L. C.Hedrick, which is owned by the assignee of the present invention.

The attenuator sections are releasably mounted in slots on the peripheryof the attenuator drum for easier replacement as shown in US. Pat. No.3,299,373 of M. Conney. However, the present attenuator apparatusdiffers from such prior apparatus by grounding the selected attenuatorsection through, at least two closely adjacent parallel current paths ofapproximately the same length for each ground point to cause the leadinductances to be lowered to a minimum, thereby reducing inductance andpreventing signal distortion due to ringing. In addition, attenuatorsections of the present invention are in the form of ceramic stripshaving voltage divider resistors coated thereon and capacitors formedintegrally therewith. A damping resistor is provided in each attenuatorsection and connected between the junction of the voltage dividerresistors and the output terminal of a coupling capacitor so that suchdamping resistor prevents ringing without affecting the DC signal.

While the step attenuator apparatus of the present invention may beemployed in any electronic circuit where it is desired to change theamplitude of a signal in predetermined steps, it is especially useful incathode-ray Oscilloscopes because of its extremely wide frequencybandwidth. This improved frequency response is the result of the lowinductance as well as a minimum input and'output capacitance and minimumground impedance of the present attenuator. The attenuator sections usedin such attenuator enable a reduction in size, lower cost and fewerparts to be assembled. The drum attenuator of the invention switches oneselected attenuator section into the circuit which isolates and shieldsthe selected attenuator section in use. Such drum attenuator alsoenables the input signals to be completely shielded from the outputsignal which greatly improves performance at high attenuation values.

It is therefore one object of the present invention to provide animproved step attenuator apparatus of small size, low cost and widefrequency bandwidth.

Another object of the present invention is to provide an improved drumattenuator in which the selected attenuator section is grounded throughat least two adjacent parallel current paths for each ground point toreduce inductance and prevent signal distortion due to ringing.

A further object of the invention is to provide an improved drumattenuator having a plurality of attenuator sections releasably mountedin slots on the periphery of the attenuator drum to enable easyreplacement of the sections and to electrically isolate or shield suchsections.

An additional object of the invention is to provide an improvedattenuator section including a ceramic strip upon which resistors areformed as coatings and having capacitors formed integrally therewith, toprovide an attenuator section of reduced size, lower cost, a minimum ofparts and with better frequency response.

Still another object of the present invention is to provide an improvedattenuator section which includes a damping resistor connected betweenthe output terminal of a coupling capacitor and the junction of a pairof voltage divider resistors to provide an improved frequency responseby reducing ringing without affecting the DC signal.

LII

' attenuator apparatus beside the drum [0 and BRIEF DESCRIPTION OF THEDRAWINGS Other objects and advantages of the present invention will beapparent from the following detailed description of a preferredembodiment thereof and from the attached drawings, of which:

FIG. 1 is an elevation view of one embodiment of the drum attenuatorapparatus of the present invention;

FIG. 2 is a side elevation view of the apparatus of FIG. I with partsbroken away to show the grounding contacts;

FIG. 3 is a view taken along the line 3-3 of FIG. 2 with parts brokenaway to show the attenuator sections and the switch contacts;

FIG. 4 is an enlarged view of a portion of FIG. 3 showing the end of oneof the attenuator sections as it is engaged by a grounding contact;

FIG. 5 is an elevation view of one of the attenuator sections of thepresent invention with parts broken away for clarity;

FIG. 6 is a partially schematic view showing the relative positions ofthe electrical components and the ground contact points on theattenuator section of FIG. 5; and

FIG. 7 is a schematic diagram of the electrical circuit of theattenuator section of FIGS. 5 and 6.

DESCRIPTION OF PREFERRED EMBODIMENT As shown in FIGS. 1, 2 and 3, thestep attenuator apparatus of the present invention includes anattenuator drum l0 having a plurality of attenuator sections 12 ofdifferent attenuator values mounted in uniformly spaced relationship onthe periphery ofsuch drum. The drum [0 is mounted on a shaft 14 whichrotates such drum between a plurality of switch positions determined bya detent mechanism 16 of conventional type. For example, l2 attenuatorsections may be employed in which case the detent mechanism [6 provides12 different attenuator positions each spaced apart by 30. The shaft 14may also be coupled to a pair of conventional rotary switches l8 whichmay be employed to actuate a switch position readout device thatprovides a visual display of the attenuator drum position.

A coaxial cable input 20 of the BNC type is secured to the adjacent to aknob 22 attached to the end of the drum actuator shaft 14. The innerconductor ofthe coaxial connector 20 has its output end 24 connected toan electrical circuit (not shown) mounted on an etched circuit board 26.The electrical circuit is a switch circuit used in an oscilloscope forswitching its vertical signal input between AC and DC coupling andground and has one output connected to a switch contact 28 providing thesignal input to the attenuator drum. through a flexible leaf springconductor 30 as shown in FIG. 3. The input switch contact 28 isinsulatingly mounted on a leaf spring 32 so that such contact extendsthrough a hole 34 in a grounded support member 36 and one end of thespring is attached to the support member by a bolt 38. Similarly, anoutput switch contact 40 is mounted on another spring attached to thesupport member 36 by means of a bolt 42 at a position longitudinallyspaced along the drum axis from switch contact 28. The output switchcontact 40 is connected by another flexible leaf spring conductor 43 toan output terminal 44 supported on a standoff post 45 of insulatingmaterial on the exterior of a shielded housing. The housing includes afirst housing portion 46 containing the input signal circuit on thecircuit board 26,

and a second housing portion 48 is provided about the attenuator drum 10so that the output signal of such attenuator drum is completely shieldedfrom its input signal. It should be noted that a pair of feed-throughcapacitor terminals 50 and 51 may be provided on housing 46 forconnecting to a relay in the switching circuit on the circuit board 26to an external source of DC operating voltage and for transmitting areadout signal corresponding to the switch position.

As shown in FIGS. 1 and 3, the attenuator sections 12 are mounted withinlongitudinal slots 52 separated by ridges or fingers 54 and uniformlyspaced about the periphery of the attenuator drum 10. Such attenuatordrum may be made of a resilient plastic material, such as nylon, so thatthe fingers 54 between such slots are resilient to enable easy insertionand removal of the attenuator sections and to hold such sections withinsuch slots. A metal clip member 56 provided on each of the attenuatorsections 12 has a pair of side flanges which engage notches in thefinger 54 on opposite sides of the slot 52 to hold such sections withinsuch slot. In addition. the metal clips 56 also function as groundplanes as hereafter discussed with reference to FIGS. 4, and 6. Theattenuator drum [0 of insulating material electrically isolates theattenuator sections from each other so that such sections are notgrounded until they are moved into the selected position where theircontacts engage the input contact 28 and the output contact 40.

The selected attenuator section is grounded by a plurality of leafspring ground contacts 58 which are formed as part of a single sheet 59of metal attached to the grounded support member 36 bolts 60 as shown inFIGS. 2 and 4. Thus one end of each spring contact 58 is fixed to thesupport member by such bolts. The intermediate portion of ground contact58 en gages a ground contact point on the grounding plane portion of theclip 56 of the selected attenuator section. while the free end of thecontact engages the surface of the support member 36 and slides acrosssuch surface during the flexing of such contact. As a result each groundcontact portion or point on the selected attenuator section is connectedto ground through two adjacent parallel current paths 62 and 64 ofapproximately the same length formed by the opposite ends of the groundcontact 58. The current which flows in opposite directions throughcontact 58 along such two paths as indicated by arrows 62 and 64 to theelectrical ground point 66, creates two magnetic fields of equalstrength and opposite polarity which tend to cancel each other and lowerthe lead inductance in such ground contact. This reduces inductance inthe attenuator sections and prevents signal distortion due to ringing,which is the oscillation of a signal at the resonant frequency of thecircuit inductance.

As shown in FIGS. 5, 6 and 7, the attenuator sections l2 each include aceramic strip 68 which is soldered or otherwise bonded to the metal clipmember 56. A pair of voltage divider resistors 70 and 72 connected inseries between an input contact 74 and the grounded clip 56. are formedas coatings of resistive material on the bottom of the ceramic strip.The common terminal of such voltage divider resistors is connected to anoutput contact 76. Both of contacts 74 and 76 are insulatingly mountedon the metal clip member by a sleeve 78 and extend through an aperturein the ceramic strip 68 so that such contacts can be connected by solderat their lower ends to the voltage divider resistors. A variable inputcapacitor 80 is formed integral with the ceramic strip so that ametallized portion of the ceramic material provides part of suchcapacitor and a partially metallized ceramic disc the other part, suchdisc being rotatable by a slotted adjustment member to vary thecapacitance. The input capacitor is connected between the input contact74 and the grounded clip 56. A variable coupling capacitor 82 is alsoformed integral with the ceramic strip and is connected between theinput contact 74 and one terminal of a damping resistor 84. The dampingresistor 84 is provided as a resistive coating on the top of the ceramicstrip 68 and has its other terminal connected to the output contact 76at the common junction of the voltage divider resistors. The dampingresistor prevents ringing by damping the signal oscillations withoutattenuating the DC signal transmitted through the voltage dividerresistors. Such damping resistor a value on the order of about 75 ohmswhen the values of the voltage divider resistors 70 and 72 are,respectively, 600 kilohms and 675 kilohms. Of course, the values of thevoltage divider resistors change for each different attenuation value,as does the value of the capacitors 80 and 82. A shunt capacitor 86 maybe employed in the attenuator section, and is connected between groundand the common terminal of coupling capacitor 82 and damping resistor84. Two pairs of grounding fingers 85 extending from the sides of theground plane 56 connect the capacitor 86 to ground. Thus capacitors 82and 86 provide an AC voltage divider while resistors 70 and 72 provide aDC voltage divider for the signal being attenuated. Input capacitor 80is adjusted to provide each attenuator section with the same inputcapacitance and compensates for changes in the values of capacitors 82and 86, as well as other capacitance.

As shown in FIGS. 5 and 6, four ground contacts 58 engage the groundclip 56 including one contact between the input terminal 74 and a hole88 provided through such clipto enable adjustment of the input capacitor80. In addition. two other ground contacts engage the clip on oppositesides of a second hole 90 provided for adjustment of the couplingcapacitor 82. The fourth ground contact is placed immediately above thefixed capacitor 86 so that is is connected to ground through the lowinductance current paths of such contact. The input and output contacts74 and 76 are separated by these last three ground contacts. As a resultthe attenuator sections of the present invention have extremely lowinductance and a wide frequency bandwidth.

It will be obvious to those having ordinary skill in the art that manychanges can be made in the above-described preferred embodiment of thepresent invention without departing from the spirit of the invention.For example, the attenuator drum 10 can be made of metal rather thanplastic, and each ground contact 58 can be divided into two contactswhose free ends engage the ground clip 56. Therefore the scope of thepresent invention should only be determined by the following claims.

I claim:

1. A step attenuator apparatus comprising:

input means for applying input signals to the attenuator apparatusincluding a fixed input switch contact;

output means for transmitting an attenuated output signal from saidapparatus including a fixed output switch contact;

a plurality of attenuator sections of different attenuation values;

carrier means for movably supporting the attenuator sections toselectively connect said sections between the input switch contact andthe output switch contact of the attenuator apparatus; and

ground means including switch contact means for selectively connectingthe selected attenuator section to an electrical ground by engagementwith at least one ground contact portion of said selected section, saidswitch contact means providing at least two parallel current pathsextending in opposite directions for each ground contact portion toreduce inductance and prevent signal distortion.

2. An attenuator apparatus in accordance with claim 1 in which the twocurrent paths are provided by the opposite ends of a single springcontact having an intermediate portion forming one end of each of saidcurrent paths to provide a common terminal, said current paths beingclosely adjacent each other and of approximately the same length.

3. An attenuator apparatus in accordance with claim 2 in which thespring contact is mounted so that one end is fixed to a support member,its intermediate contact portion releasably engages said one groundcontact portion of the selected attenuator section, and its other endslidably engages said support member.

4. An attenuator apparatus in accordance with claim 3 which includes aplurality of other spring contacts whose intermediate contact portionsengage a plurality of other ground contact portions on said selectedattenuator section.

5. An attenuator apparatus in accordance with claim 1 in which thecarrier means includes a rotary drum having longitudinal slots providedin its outer surface and mounting means for releasably mounting theattenuator sections in said slots.

6. An attenuator apparatus in accordance with claim 5 in which the drumis made of resilient plastic material which insulates the attenuatorsections from each other so that only the selected section is grounded.

7. An attenuator apparatus in accordance with claim 6 in which theattenuator sections each include a ceramic strip on which resistors andcapacitors are provided to form the attenuator section and a metal clipmember issecured to said ceramic strip to mount such section in the drumslots and to provide a ground plane for said section on which the groundcontact portion is located.

8. An attenuator apparatus in accordance with claim 4 in which the inputand output switch contacts are resiliently mounted on the support memberin holes positioned between said spring contacts.

9. An attenuator section comprising:

voltage divider means including a pair of resistors connected in seriesbetween an input terminal and ground, and an output terminal connectedto the common connection of said pair of resistors; an input capacitorconnected between said input terminal and ground;

a coupling capacitor having one terminal connected to said inputterminal; and

a damping resistor connected between the other terminal of said couplingcapacitor and said common connection a strip of ceramic material onwhich all of said resistors are provided as coating of resistivematerial and which also forms part of said input and couplingcapacitors, and a metal ground plane member to which said strip issecured.

10. An attenuator section in accordance with claim 9 which also includesa shunt capacitor of fixed value connected between the other terminal ofthe coupling capacitor and ground.

11. An attenuator section in accordance with claim 9 in which the inputand coupling capacitors are variable and the ground plane member isprovided with apertures therethrough in alignment with such capacitorsto enable adjustment thereof.

1. A step attenuator apparatus comprising: input means for applyinginput signals to the attenuator apparatus including a fixed input switchcontact; output means for transmitting an attenuated output signal fromsaid apparatus including a fixed output switch contact; a plurality ofattenuator sections of different attenuation values; carrier means formovably supporting the attenuator sections to selectively connect saidsections betweeN the input switch contact and the output switch contactof the attenuator apparatus; and ground means including switch contactmeans for selectively connecting the selected attenuator section to anelectrical ground by engagement with at least one ground contact portionof said selected section, said switch contact means providing at leasttwo parallel current paths extending in opposite directions for eachground contact portion to reduce inductance and prevent signaldistortion.
 2. An attenuator apparatus in accordance with claim 1 inwhich the two current paths are provided by the opposite ends of asingle spring contact having an intermediate portion forming one end ofeach of said current paths to provide a common terminal, said currentpaths being closely adjacent each other and of approximately the samelength.
 3. An attenuator apparatus in accordance with claim 2 in whichthe spring contact is mounted so that one end is fixed to a supportmember, its intermediate contact portion releasably engages said oneground contact portion of the selected attenuator section, and its otherend slidably engages said support member.
 4. An attenuator apparatus inaccordance with claim 3 which includes a plurality of other springcontacts whose intermediate contact portions engage a plurality of otherground contact portions on said selected attenuator section.
 5. Anattenuator apparatus in accordance with claim 1 in which the carriermeans includes a rotary drum having longitudinal slots provided in itsouter surface and mounting means for releasably mounting the attenuatorsections in said slots.
 6. An attenuator apparatus in accordance withclaim 5 in which the drum is made of resilient plastic material whichinsulates the attenuator sections from each other so that only theselected section is grounded.
 7. An attenuator apparatus in accordancewith claim 6 in which the attenuator sections each include a ceramicstrip on which resistors and capacitors are provided to form theattenuator section and a metal clip member is secured to said ceramicstrip to mount such section in the drum slots and to provide a groundplane for said section on which the ground contact portion is located.8. An attenuator apparatus in accordance with claim 4 in which the inputand output switch contacts are resiliently mounted on the support memberin holes positioned between said spring contacts.
 9. An attenuatorsection comprising: voltage divider means including a pair of resistorsconnected in series between an input terminal and ground, and an outputterminal connected to the common connection of said pair of resistors;an input capacitor connected between said input terminal and ground; acoupling capacitor having one terminal connected to said input terminal;and a damping resistor connected between the other terminal of saidcoupling capacitor and said common connection, a strip of ceramicmaterial on which all of said resistors are provided as coating ofresistive material and which also forms part of said input and couplingcapacitors, and a metal ground plane member to which said strip issecured.
 10. An attenuator section in accordance with claim 9 which alsoincludes a shunt capacitor of fixed value connected between the otherterminal of the coupling capacitor and ground.
 11. An attenuator sectionin accordance with claim 9 in which the input and coupling capacitorsare variable and the ground plane member is provided with aperturestherethrough in alignment with such capacitors to enable adjustmentthereof.